And there appears to be little farmers can do to avoid coming under drone surveillance - flying drones above tree height is legal.

''Our legal advice is that if you're no nearer than 10 metres above ground, and you're not filming in anyone's houses, you can go ahead,'' said Mark Pearson, head of the animal welfare group.

''For example, if an egg producer says that they are free range, it would be helpful to check their claims by filming from above the property. You can gather the evidence, and there's no need to trespass. Or let's say we find a sheep dying from fly strike, we can record the location on a GPS and notify the authorities,'' he said.

The group bought the six-bladed, helicopter-type drone for $14,000 from a commercial supplier, using public donations, and has just completed a training program. Deployment will begin next week, with several farms and businesses earmarked for surveillance.

Farmers were dubious about being watched by drones.

''Many people in rural communities would see this as another attack on their peace of mind and an invasion of their privacy,'' said president of the NSW Farmers Association Fiona Simson.

She said farmers recognised that safe food meant healthy and happy animals. ''NSW Farmers does not condone any acts of animal cruelty and farmers are committed to high animal welfare standards,'' Ms Simson said.

Mr Pearson said the drone would not just be used to gather evidence of illegal cruelty, but would also film some routine, legal farm practices that might upset non-farmers. ''We're not interested in what farmers may be doing in their daily activities, and I completely respect people's privacy,'' he said.

''But there are lots of cases where farming activities cause horrible distress to animals - mulesing being a common example. People are entitled to know and see what's going on. So, even if it is lawful, if we think the public is going to be outraged or if we think they need to be informed, we will show it.''

This month Animal Liberation exposed horrific acts of cruelty at an Ingham turkey farm in Sydney, after anonymous footage was handed to the group showing turkeys being bashed and trodden on.

Australian Privacy Commissioner Timothy Pilgrim has said ''the potentially intrusive nature of the technology'' meant there should be a public debate about existing regulations.